This invention pertains generally to DC pulse generators having a substantially constant energy pulse output and more specifically to such generators that can be employed in combination with radioisotopic thermoelectric generators in cardiac pacemaker applications.
The use of implanted battery powered pacemakers has become a common medical procedure for alleviating the effects of Stokes-Adams disease. Persons having this disease exhibit an abnormally slow pulse rate due to the breakdown of the body's natural heart stimulating mechanism. By electrically stimulating the heart with very low energy electrical impulses, it can be made to resume a normal pulse rate and thus the patient can lead a more normal life. The use of battery power has presented a major drawback in pacemaker design due to its finite storage capacity. Surgical replacement is required at intervals ranging between six months and three years for most pacemakers. Moreover, the output pulse energy must be kept as small as possible to minimize battery drain and this sometimes allows little margin between the available pulse energy and the minimum exciting threshold of the heart. Obviously, a better power source is needed.
Radioisotope thermoelectric generators similar to those employed in spacecrafts for long duration power sources are a desirable alternative for this application. A decaying radioisotope material produces heat which is converted by a thermoelectric element into a low voltage power source. Such a system will operate for ten years with only about a ten percent drop in power output, thus exhibiting a dramatic advantage over the battery sources conventionally employed.
There are, unfortunately, some limitations to employing radioisotope thermoelectric generator systems. Probably, the most significant limitation is an economic one. There is a limited source of the necessary isotope available and thus its cost is the major factor in the cost of the pacemaker. The ultimate number of such units which can be manufactured is also limited in view of the rarity of the isotope. Accordingly, it is desirable to optimize the pacemaker energy conversion to achieve an acceptable design. Presently there are several radioisotope thermoelectric generator pacemakers being manufactured, however their efficiencies are extremely low ranging between sixteen and nineteen percent. Accordingly, a more efficient pacemaker is required if radioisotopic sources are to be continually employed in pacemaker applications.
The output pulser in most pacemakers is nearly identical in operation and accounts for a major part of the system's loses. Conventionally, pacemakers charge a 3.3 microfarad capacitor to six volts, which is subsequently discharged into the heart. The capacitor is completely emptied each pulse thus precisely metering sixty microjoules of energy into the heart independent of the impedance of the output load. Each time the capacitor is recharged, the charging resistance dissipates as much energy as the capacitor receives, resulting in a fifty percent system charging loss. An apparent alternative is to apply a fixed voltage square wave pulse to the load every cycle and thus avoid complete capacitor recharging. This is undesirable from a medical viewpoint since the output energy per pulse would then vary inversely to the impedance of the output load, and the heart's impedance, while normally three hundred ohms, can easily vary from one hundred to one thousand ohms. Accordingly, a new approach to the output pulser is required if an overall improvement in efficiency is to be obtained.